The slick, numbingly relentless new film version of “It,” adapted from the 1986 Stephen King best-seller and a lot rougher than the 1990 TV miniseries, gets a few things right, in flashes of imagery and in the performances. The opening scene is brutally effective, depicting the little Derry, Maine, resident Georgie meeting his cruel preteen doom at the hands, and teeth, of the malevolent supernatural clown Pennywise, and then dragged at alarmingly high speed down into the sewer.

Director Andy Muschietti (born in Argentina, previously known as Andres Muschietti) knows the visceral cinematic value of something wicked this way coming at you, very quickly, herky-jerky style. That was the key to his splendid little short film “Mama” from 2008, which was then expanded into a 2013 feature. Going into “It,” I hoped we’d get more of that visually suggestive fright. It’s there in a few shots: the initial glimpse of the floaters down below, for example, or the slide carousel running amok and then springing Pennywise, played with formidable, unblinking glee by Bill Skarsgard, off the projection screen and into the faces of the kids he’s trying to scare to death.

Those kids are played by some skillful young actors, notably Jaeden Lieberher (“St. Vincent”) as the anguished protagonist Bill. In the prologue his brother Georgie (Jackson Robert Scott) slips out of his life, leaving only a puddle of blood in the street. Bill’s makeshift gang known as “the loser’s club” constitutes a familiar, King-style band of bullied, abused, marginalized teenagers. It’s “Stand by Me” with a killer clown, a shape-shifting, endlessly versatile scare mechanism tailored to each character’s worst fears. Sophia Lillis plays Beverly, the boys’ lust object, whose incestuous father (Stephen Bogaert, always falling asleep in front of the TV like every bad parent in “It”) has prepared her for evil in many forms. Lillis and Lieberher keep the emotional stakes as high and honest as possible.

The screenplay credited to Chase Palmer, Cary Fukunaga (originally set to direct) and Gary Dauberman shaves King’s massive book roughly in half. In this two-hour, 15-minute picture, we’re dealing only with the kids (transplanted from the 1950s to the 1980s), not their adult selves. The sequel promised by the movie’s finale will take place 27 years later.

That narrative change works fine in principle. The larger question is one of rhythm, and the diminishing returns of one jump scare after another. Director Muschietti’s film is afflicted by a weird case of clutter; nearly every scene begins and ends the same way, with a slow build, a vulnerable child in a cellar or an old, dark house, a violent, bloody confrontation (either in the everyday bullying sequences, which are psychotically vicious, or in the Pennywise appearances) leading up to a KAAA-WHUMMMMMM!!!! sound effect. Such familiar tactics will likely ensure a healthy box office return (the movie’s expected to make $70 million opening weekend), but the result plays like an Olympic hurdles event, with a really, really long track.

King knows what he’s doing: Back in 1986, the year “Stand by Me” came out in theaters, “It” put the whammy on millions. He couldn’t lose. Sinister red balloons. The geyser of blood gushing up from the bathroom sink. Coulrophobia, the fear of clowns, is money in the bank, as well as a tiring cliche, one the World Clown Association takes seriously. From their recent, sternly worded protest letter: “People dressed as horror clowns are not ‘real clowns.’ They are taking something innocent and wholesome and perverting it to create fear in their audience.”

King was hardly the first to exploit that fear factor. The movie won’t be the last. While Pennywise has been given a fabulous costume (thanks to designer Janie Bryant), and cinematographer Chung-hoon Chung’s smudged interiors and not-quite-blue skies (until the final scene) do their part, what do we have here, really? We have a story that feels not so much freshly imagined as dutifully recounted.